Just looked them up on the NWNWiki. Lots about their abilities, but not weaknesses. They (and the succubi) keep killing me at a conflict in Neverwinter. All that magic- and elemental resistance! I need to take them down rapidly or else my squad is worn down.

I have Sand, Qara and Casavir. My toon is a strength-based monk who only seems to distract them. Thinking of replacing Qara with Elanee (for the meat-shield animal companion) and Sand with Khelgar to deal out massive damage with a two-handed weapon. Only use for Sand would be casting Dispel Magic to remove their defenses, but Blind Fight on Khelgar is probably as effective.

EDIT: Removed possible spoilers

This is in NWN2.

As far as I can find it works like this:

Chaotic Evil (demons) -- Vulnerable to Cold Iron weapons (eg Succubi)
Lawful Evil (devils) -- Vulnerable to Alchemical Silver weapons (eg Erinyes)

I equipped Casavir and Khelgar with two-handed weapons in both materials, and the fight was much easier. Swapped out weapons depending on what they were fighting, but not sure it was materials so much as the fact that they went in two-handed.

Stoneskin and Globe of Invulnerability for Sand kept him up and going. My level 12 monk couldn't do much damage even with +3 gloves and 2 sonic damage, but mostly managed to avoid being hit. I used her to distract troublesome opponents who were giving the other party members a hard time. Very effective on mephits .. the shame of it!

More potions of Speed and Dispel Magic would have helped a lot. Anyone know where I can buy Healers Kits? At the moment I'm going back to Highcliff and all I get there are +1. Plus it rather breaks the immersion.

EDIT: To distinguish demons and devils alignments